In the technical sector relating to the manufacture of gearwheels, the “classic” method of cutting the teeth of gearwheels envisages using special tools mounted on dedicated machines, called gear-cutting machines. Usually the gear-cutting machines perform a first—so-called prefinishing—operation, for cutting the teeth of the gearwheel, and a second—so-called finishing—operation. Between the first and second operation usually a procedure for heat treatment of the gearwheel is performed.
When using a gear-cutting machine, the prefinishing is performed using a shaped tool, usually called a roughing hob which cuts the teeth from the blank by rotating the gearwheel on the hob (which is in turn rotationally synchronized), while the finishing operation is performed using another shaped tool, usually called a finishing hob, which performs a finishing operation on the final profile so as to form the final profile of the gearwheel with the desired surface finish. The difference in radial penetration between the roughing tool and the finishing tool is usually limited to about a few tenths of a millimeter, apart from the bottom of the tooth, where the finishing operation preferably does not remove any material. This ensures a greater strength of the finished gearwheel and prevents frictional contact between the tip of the hob tooth and the bottom of the tooth space, thereby reducing greatly the stress and wear on the end edges of the hob teeth. This ultimately improves the precision and quality of the finished surface of the gearwheel.
US 2011/268524 for example describes the use of milling cutters shaped with the exact profile of the space to be obtained between two adjacent teeth, with the roughing cutter which reaches as far as the bottom of the space which is to be obtained between the teeth and the finishing cutter which has a smaller diameter so as not to touch the bottom which has already been machined.
U.S. Pat. No. 5,136,522 describes a machine which cuts a gearwheel by means of a single milling cutter specially shaped according to the space to be obtained between the teeth. A sensor is able to sense the space between the teeth and generate a correction signal.
Obviously, with the classic method mentioned above it is necessary to provide a number of gear-cutting tools corresponding to the number of profiles of the racks which are used to generate the geometric profiles of the gearwheels which are to be made.
The system is very efficient in the case of production on a large scale and the market for these systems is far-reaching, there existing various computerized programs for designing the gears, which perform calculation of the machining curves followed by the pair of gear-cutting tools, as well as, for example, simulation of the gearwheel so that its characteristics may be precisely defined before actual manufacture.
For the production of gears on a small scale or in very small numbers, the realization thereof by means of dedicated cutting tools is however excessively costly. In such cases, therefore, for some time it has been proposed cutting the gearwheel by means of numerical-control machine tools which are advantageously of the 5-axis type. According to the customary procedure, in the case of these machines a mathematical representation of the surface which is to be obtained is created and the machine follows this surface with its own cutting tools, thus forming the desired part.
When manufacturing gearwheels, however, the advantages of having a large number of programs for designing gearwheels by means of gear-cutting machines, already used for some time and well-known to gearwheel manufacturers who use the “classic” method, are lost.
DE 10 2005 05054513 describes a gearwheel formed by means of machining a continuous rough-formed surface and a continuous finishing surface. The dimensions, described in this patent application, of the continuous finishing surface compared to the continuous rough-formed surface are said to improve meshing of the gears. The problem, however, of how to optimize machining of the two surfaces calculated is not considered in any way.
EP 2,314,404 describes a method for moving a tool along parallel lines in order to obtain finishing of single surfaces of a gear which has already been cut.